Steel sheet intended for the manufacture of parts for a terrestrial motor vehicle is generally coated with a zinc-based metal layer for corrosion protection, deposited either by hot-dip coating in a zinc-based liquid bath or by electrodeposition in an electroplating bath containing zinc ions.
Galvanized sheet intended for the manufacture of shell parts then undergoes a forming operation and is assembled to form a body-in-white, which is then coated with at least one coat of paint, thereby providing greater corrosion protection and an attractive surface appearance.
For this purpose, conventionally, automobile manufacturers firstly apply a cataphoretic coating to the body-in-white, followed by a primer coat of paint, a base coat of paint and optionally a varnish coat. To obtain a satisfactory painted surface appearance, it is general practice to apply a total paint thickness of between 90 and 120 μm, consisting of a cataphoretic coating 20 to 30 μm in thickness, a primer coat of paint 40 to 50 μm in thickness and a base coat of paint 30 to 40 μm in thickness, for example.
To reduce the thickness of paint systems to less than 90 μm, certain automobile manufacturers have proposed either to dispense with the cataphoresis step or to reduce the number of coats of paint in order to increase productivity. However, at the present time, this thickness reduction of the paint system is always to the detriment of the final appearance of the painted surface of the part and is not implemented in industrial production.
The reason for this is that the surface of the zinc-based coatings serving as base substrate has what is called a “waviness” which, at the present time, can be compensated for only by thick coats of paint under penalty of having what is called an “orange peel” appearance, which is unacceptable for body parts.
The waviness W of the surface is a slight pseudoperiodic geometrical irregularity with quite a long wavelength (0.8 to 10 mm) which is distinguished from the roughness R, which corresponds to geometrical irregularities of shorter wavelengths (<0.8 mm).